It is known for vehicle owners to modify the engine control system of their vehicles in order to increase the maximum output power of the engine. One method of increasing the maximum output power at a specific engine speed of a common rail equipped diesel engine involves modifying the engine control system so that a false reduced rail pressure value is provided to the electronic control unit (ECU) of the engine. The modification causes a closed loop control strategy of the ECU to increase the fuel pressure in the common rail so that the false reduced rail pressure equals the demanded rail pressure. The result of this is that the actual pressure within the common rail is greater than the pressure reported to the ECU, which is the false reduced value. As the quantity of fuel delivered in a fixed period is a function of fuel pressure, the modification at maximum driver demand causes excess fuel to be delivered to the combustion chamber, resulting in increased engine torque and hence increased output power of the engine.
An increase in engine torque or rail pressure beyond that for which the engine is designed may cause increased engine wear and result in product failure. In turn, this may lead to warranty claims against the vehicle, engine, fuel injection system or other component manufacturers. It is therefore an aim of the present invention to provide an improved method of controlling a vehicle engine system to prevent unauthorised modification of engine power.